1. Field of Invention
The present invention relates to a boost driver circuit with fast discharging function. Particularly, the present invention discloses a boost driver circuit using less number of high voltage transistors; the driver circuit for example can be used as a driver circuit for Light Emitting Diode (LED).
2. Description of Related Art
One application of a boost converter is to drive LEDs. Referring to FIG. 1, the LEDs connected in series require a relatively high voltage, and thus it requires to convert a lower input voltage Vin to a higher output voltage Vout for the LEDs. The conventional LED driver circuit 1 as shown comprises a pulse width modulation (PWM) driver circuit 14, whose output signal controls a power transistor M1. The power transistor M1 switches to convert the input voltage to the output voltage. The LED driver circuit 1 usually further comprises an under voltage lock-out (UVLO) circuit 12 and an over voltage protection (OVP) circuit 16 for overriding the operation of the PWM circuit 14 when the input voltage is lower than a threshold value or the output voltage higher than a threshold value to avoid circuit damages. Moreover, the LED driver circuit 1 further comprises a current regulator circuit 18 cooperating with a corresponding transistor M2 to regulate the current passing through the LED.
In the foregoing circuit, the transistor M1 must be a high voltage transistor because it faces the high voltage at the node LX. In addition, the transistor M2 must be a high voltage transistor as well, because at the instant when the power of the LED driver circuit 1 is turned off, the feedback node FB will be instantly pulled up to the output voltage Vout; therefore, the drain of the transistor M2 should be durable for high voltage, to absorb a large amount of sudden current and conduct the charges to ground. However, the high voltage transistor costs a substantial circuit area, causing difficulties for layout; thus, the conventional circuit is not ideal.
Accordingly, the present invention provides an LED driver circuit with fast discharging function which uses less number of high voltage transistors to overcome the drawbacks of the foregoing prior art.